Insects & Diseases

Information below was created by Colorado State University Extension Coop

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Purple Needles on Spruce Trees

By Mary Small, Colorado State University Cooperative Extension, Urban Integrated Pest Management

The appearance of purple spruce needles usually points to root dehydration. If the damage appears during the winter or early spring, it’s probably the result of winter injury.

All spruce trees, but especially those growing in or near lawns, need water during the dry fall and winter months. This keeps the needles hydrated and healthy. Other factors can dehydrate spruce roots and should be considered when diagnosing the problem. For example, de-icing salts and excess fertilizer can also cause or contribute to the off-color because they dehydrate roots. A girdled root cuts off or reduces the amount of water reaching the needles, causing purple needles. Any activity that damages spruce roots (like trenching or digging) also prevents them from absorbing water well, producing the off-color

Cooley Spruce Gall Aphid (Adelges cooleyi)

By Whitney Cranshaw, professor and extension specialist in entomology, Colorado State University, 1998

Where you will find it: The Cooley spruce gall aphid distorts the terminal growth of spruce. This abnormal growth, or “gall”, appears cone-like and is a light brown color after weathering. Galls are found in greatest number on the north and east sides of the tree. Incidence of galling has been unusually abundant over the past couple of years along much of the Front Range.

Habits of the Cooley spruce gall aphid: The insect is a type of “woolly aphid” that covers itself with white waxy threads. During the winter and early spring it is on the underside of spruce twigs, usually within a couple of inches of the buds. Just pinhead size through the winter, it resumes feeding and swells rapidly during late April and May. Ultimately the females produce a large mass of eggs, which hatch in synchrony with bud break.

The young aphids move to emerging needles and start to feed. They suck a bit of sap but also inject substances in their saliva that induce the new growth to swell and overgrow them. Protected by the plant tissues, they continue to grow within sells of the gall, becoming full grown in late spring. At this time the galls begin to dry out and crack open, and the Cooley spruce gall aphids crawl out on the needles. With one more molt they transform to a winged stage that leaves the tree permanently. A Douglas fir is their ultimate destination, their “alternate host”. However, no galls are produced on Douglas fir and damage is slight. At the end of the growing season a few new winged forms will leave the plant and colonize spruce to establish new infestations.

Control of Cooley spruce gall aphid: This is almost entirely a cosmetic problem, so there is little need for controls to protect the health of the plant. However, many find the galls unattractive. The insect is easily managed by spraying the underside of twigs to kill the overwintering stages in spring before bud break Carbaryl, permethrin and horticultural oils are effective for control of Cooley spruce gall.

There is considerable variation in susceptibility of individual spruce trees to this insect. Some appear to have fairly chronic problems. Other trees, particularly many with a deep blue coloration, seem to have fair resistance.

Removing old galls has no effect on Cooley spruce gall management, as the insect has already migrated from the galls and they are never again occupied. Such removal may be useful if one is bothered by their appearance. Left alone they are buried by subsequent new growth and are little visible after a year or two.

Controlling Aphids on Trees

By Whitney Cranshaw, Colorado State University Cooperative Extension Specialist, Entomology

Most aphids on trees and shrubs have complicated life cycles, switching between different plants during the growing season. This habit, plus their phenomenal ability to reproduce and their great sensitivity to myriad natural controls, causes great fluctuations in their numbers from season to season. Very little correlation between problems exists from one season to the next.

Aphids that were present on trees during autumn lay eggs near buds. A spray of horticultural oil before bud break can kill these eggs, and this will help prevent aphid problems in the spring. This, however, will have little effect on subsequent fall aphid problems, which are dependent on conditions during summer and fall. At best, certain persistent insecticides that move systemically in the plant may provide control through the fall. Imidacloprid (Bayer Advanced Garden Tree and Shrub Insect Control) is a newly available systemic insecticide that can provide aphid control on trees for several months following application to the soil.

Scale Insect Pests

By Whitney Cranshaw, Colorado State University Cooperative Extension, Entomology

  • They may be little but they can be mighty
  • They are scales, one of the most insidious insect pests affecting landscape plants in Colorado. They are tiny, typically inconspicuously colored and largely immobile. They spend their lives quietly removing sap from trees and shrubs.
  • Several important scale insects, however, can cause serious damage, causing dieback of branches and occasionally killing the plant.
  • Scales are close relatives to the more familiar aphids. The main difference is the presence of the plate-like scale covering, a waxy material secreted through pores on the back. This covering gives the insects substantial protection from the environment, as well as from many of the control sprays we might want to use. Because of this, scale insects often are one of the more difficult pests to manage.
  • Dozens of scales can occur. The knowledgeable gardener will become familiar with a few important ones.


Oystershell scale



  • The most significant of these pests is the oystershell scale, so named because it resembles a minute oyster attached to the bark of trees. Aspen, ash, lilac, cotoneaster, willow and poplars are among the many plants in the region highly susceptible to this insect. Oystershell scales kill, often because of the increased susceptibility to disease caused when the scales sap the plant’s vigor.


Scales of pine and spruce



  • Pine and spruce can be affected by two common scales. Pine needle scale is an elongate, white insect that attaches itself to the needles of the spruce and several pines, notably mugho. This insect, occurring in great numbers the past few seasons, has resulted in premature needle drop on pine and spruce.
  • A more recently introduced pest in the state is the pine tortoise scale, a “soft” globular-shaped scale that occurs on the twigs of Scots pine and a few other pine species. Pine tortoise scale also produces large amounts of sticky honeydew, which attracts nuisance visits by yellowjackets and honeybees.


Control of scales



If scales are damaging your plants, you might need to take several approaches to control. For oystershell scale on small trees and shrubs, a simple and highly effective practice is to gently scrape the scales off the trunks and branches with a soft, plastic scrub pad. Once dislodged from the protective covering, the eggs soon die.

Horticultural oils also are useful for

The trick is properly timing the treatment because the crawler period is brief and soon is followed by the secretion of the protective scale covering. Below are some general guidelines to help you know when crawlers emerge in the Denver Metro area:

  • Pine needle scale Late April to early May (with a second generation in mid-to-late July)
  • Oystershell scale Mid-to-late May
  • Pine tortoise scale Early-to-mid June
  • European elm scale Mid-to-late June
  • Each season can differ, however, and crawler emergences varies across the state. The best way to determine emergence is by sampling infested plants. Do this by shaking branches that contain living scales and eggs over a piece of paper; examine for the presence of minute crawlers. Alternately, on some plants, you can capture crawlers by placing a piece of double-sided sticky tape on the branch controlling many scale insects. These are specialty oils refined to allow their use on plants and are sold in many garden centers under various trade names. “Dormant oil,” “supreme oil,” “superior oil” and “spray oil” are among the descriptions. Mixed with water to a 1 to 3 percent dilution, the oils cover the insects and smother them. Most oils are used during the dormant season, but some of the newer oils allow use after leaves have emerged. (Uses and precautions are on the label of each product. Read carefully before use.)

A well-timed “crawler spray” often is the most effective way to control scales. Newly hatched scale insects are called crawlers because during this brief period they are unarmored and mobile. In this state, they are susceptible to sprays of most insecticides. Common crawler treatments include Sevin, Orthene, malathion, insecticidal soaps and horticultural oils.

Ips Beetle

By Joe Julian, Colorado State University Cooperative Extension, Horticulture

  • The recent drought has caused an increase in problems associated with the Ips beetle, a small bark beetle that attacks trees that are under some type of environmental stress, have been mechanically damaged, or recently transplanted. Ips is a native insect and common in Colorado forests. Various species of Ips attack spruce and pine in the Rocky Mountain region. The Ips beetle very rarely attacks healthy trees, although when beetle numbers build up, healthy trees may be at risk.
  • There are eleven species of Ips beetle found in Colorado. Each species is host specific, attacking only certain types of spruce or pine. One aspect of the Ips beetle that is different than the mountain pine beetle is several generations of Ips can occur in a season. However, unlike mountain pine beetle, infestation by Ips beetle does not necessarily mean the whole tree will die. The attacked tree, however, is more susceptible to additional attacks and the tree may ultimately die from the series of attacks.
  • Another factor aiding the increase of Ips beetle populations in Colorado is the creation of freshly cut wood from forest homeowners that experienced fire damage or that are trying to reduce wildfire hazards. Ips beetles will breed in the cut wood and thus population numbers increase. Adult Ips beetle will over winter under the bark of dead and dying trees or in firewood slash or woodcut to clear a fire path.
  • Controlling Ips beetle requires promotion of vigorous tree growth, something difficult to accomplish in a forested area during drought. Landscaped trees should be given adequate water. Avoid injury to roots by mechanical damage, compaction, etc. Chipping and debarking can eliminate surviving Ips beetle larvae. High valued trees should be preventatively sprayed with either carbaryl (Sevin) or permethrin. Bark beetle applications at the labeled rate should provide at least three months control of Ips beetle. It should be noted that adult Ips beetles have been observed entering trees during warm days as early as late February and on into November. Two preventative insecticide treatments may be needed to protect valuable trees during our current drought period.

Zimmerman Pine Moth

By Carl Wilson, Horticulturist, Denver Cooperative Extension

Dead and dying branches in the upper half of pine trees are a sign of Zimmerman pine moth infestation. Austrian pines are most commonly infested, although Scots and ponderosa pines are also damaged. This insect has become established along the Front Range in recent years.

Infested branches typically break at the crotch where they join the trunk (see above).

  • The first external sign of injury is the production of popcorn- like pitch masses at wound sites. The size of golf balls, masses resemble clusters of small, pale colored grapes.
  • Zimmerman pine moth overwinter as young caterpillars in cocoons under bark scales. They tunnel into trees in late-April and early May. By late spring, they move down to tunnel at the base of branches created the pitch masses.
  • Pitch masses
  • By July, larvae reach full size and pupate within the pitch masses. Adults emerge from cocoons in late July and August. Eggs are laid that hatch into the caterpillars that overwinter under the bark.

The moth is most vulnerable during late summer and again in spring when they are exposed and active on the bark. A drenching trunk spray applied in August and/or mid-April should kill larvae before they enter trunks. Astro has provided a high degree of control in Colorado State University tests.

Aspen Leaf Spots

By Laura Pottorff, Colorado State University Cooperative Extension agent, horticulture and plant pathology

As aspen trees begin to leaf out, few of us think about disease problems. But, if your tree has had Marsonnina leaf spot or rust disease in the past, bud break may be the time to use preventative sprays. Here, however, are some considerations before reaching for the fungicide bottle:

  • Be sure last year’s black or orange spots actually were caused by disease. Aspen leaves turn black or yellow or drop off for many reasons — most of them from poor growing conditions. If Marssonina or rust is present, symptoms will appear in early August to late September. Rust will cause small yellow or orange spots, while Marsonnina will cause round black spots. If these symptoms occur year after year, fungicide application might be warranted. If symptoms don’t occur every year, fungicides usually are not necessary.
  • Think about what other types of control strategies you could use in place of or in conjunction with fungicides. Fungicides are widely overused in urban areas and even when their use is justified, they may need a little help. You can minimize Marssonina and rust diseases by cleaning up infected leaves that drop to the ground in the autumn. The fungus overwinters on this leaf debris and if we clean up as much debris as possible, less “inoculum” will be present to begin the disease cycle over again in the spring.

Winter Weather Brings Sunscald to Trees

By Robert Cox, Colorado State University Cooperative Extension Agent, Horticulture

What should Mr. Smith think if a horticulturist tells him his tree has “sunscald”?

Should he use a l5-rated sunblock on the tree trunk? Should he blame the hot June weather? Make a trip to the pharmacy for antibiotics?

The answer is none of the above.

Sunscald is a winter time injury to tree trunks, caused by the sun. Deciduous trees are without leaves, night temperatures often fall below freezing and the winter sun is low in the sky. These factors combine to cause sunscald.

Here’s how it happens: Assume a young, thin-barked tree is warmed on a sunny day in January. The sun is at a low-angle and it warms the south and southwest sides of the trunk, causing inactive, cold-hardy bark cells to “think spring.” Then the sun sets and the temperature drops below freezing, killing the bark cells. Water flow from roots to the tree top is cut off, because dead cells in the trunk cannot conduct moisture. As a result, much of the tree top dies back, and the tree becomes susceptible to other organisms, such as fungi and insects. These culprits will be blamed later for the tree’s demise, but sunscald was the initial cause. Weakened plants are more susceptible to secondary organisms.

Young, thin-barked trees are most at risk. These include honeylocust, willow, mountain ash, fruit trees, maples and ashes. You can prevent sunscald. Try shading the tree through the winter by placing an upright board on the south side of the tree near the trunk.

You also could use plastic coils sold as rabbit guards. These, however, usually are only large enough to cover a portion of the trunk, leaving higher parts unprotected. Additionally, if the guards are left on too long, the plastic injures the growing tree.

The best methods involve reflecting the sunlight or insulating the trunk. For years, orchardists have used white latex paint on fruit trees to combat sunscald. This is an acceptable orchard practice, but it may look objectionable in the home landscape.

Use of crepe paper to insulate the trunk is the best method. This tree wrap, available at local garden centers and nurseries, keeps trunk temperatures cool. Start wrapping at the base of the tree, overlapping one-third with each turn. This ensures the wrap will shed water. Wrap up to just above the second branch and secure with stretchable tape. Do this in November and remove wrap around Easter. It is imperative that the wrap be removed in the spring; if left on it can harbor insects or disease and the tape can injure the tree as it expands in the spring. You’ll need to wrap the trees for the first two or three winters.

Sunscald is less likely to be a problem for trees planted on the east or north sides of buildings, because they are less likely to be in full sun than are trees planted on west or south exposures.

Sunscald isn’t a summer problem, because leaves protect the tree trunk from sun, the sun is higher in the sky and freezing temperatures aren’t likely